Propellant powder making process



. grains.

United States Patent 3,201,500 PROPELLANT POWDER MAKING PROCESS Don W. Ryker, Alton, 11L, assignor to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Filed Nov. 16,1951, Ser. No. 256,827

- 6 Claims. (Cl. 264-3) This invention relates to a method for making explosives and more particularly to an improved method for forming giant propellent grains.

Giant propellent grains having a diameter in some instances as great as two feet or more and having a length of several feet are required by the military for certain purposes such as, for example, for jet-assisted take-off rockets for aircraft or boosters for guided missiles. Heretofore, the manufacture of large propellent grains has generally required an extrusion process wherein gelatinized sheets of the explosive composition are expressed through suitable dies. Several disadvantages are inherent with this type of process; for example, huge, expensive presses capable of exerting extremely high pressures are required to express the highly viscous composition through the die, the extrusion process is slow, utmost precaution must be taken to reduce the hazard involved when subjecting explosive materials to the pressure levels required for satisfactory extrusion of the large mass necessary for forming giant grains of propellant and only limited grain sizes can be formed.

Another method for forming giant propellent grains was disclosed by Silk et al. in US. Patent 2,417,090 granted March 11, 1947. In accordance with that process, pellets of a nitrocellulose-nitroglycerin composition are treated with a plasticizerto render the surface thereof tacky and are consolidated into a homogeneous grain by means of pressure and heat. This process has the disadvantage of requiring the combination of heat and pressure to consolidate the pellets into a homogeneous grain.

It is, therefore, an object of this invention to provide a process for forming giant grains of propellent powder which is devoid of the disadvantages inherent with prior art processes. It is another object of this invention to provide a novel method for forming grains of propellent powder. It is a further object of this invention to provide an improved process for casting giant propellent Still another object of this invention is to provide an improved method for forming single base propellent grains into larger homogeneous double base propellent grains.

The foregoing objects as well as others which will be- .come apparent from the following description are accomplished in accordance with this invention, generally speaking, by providing an improved method for forming propellent grains from powder grains having lesser dimenameter of about 0.03 inch and cut to lengths substantially equal to their diameter are suitable for casting large grains up to several feet in length. The gelatinized powder grains formed in this manner, after solvent removal, are suspended in a water medium containing a protective colloid and while thus suspended are surface treated with a substantially water-immiscible nitrocellulose solvent to form a glazed surface thereon. In this treatment,

'thesurface of the grain is attacked by the solvent and 3,201,500 Patented Aug. 17, 1965 upon removal of the solvent therefrom, the surfaces of the grains are case hardened or in other words, rendered more resistant to solvent attack. After the grains have been separated from the water medium, they are charged to a suitable molding container, densified into a compact mass and the air within the voids between the grains displaced with a casting solvent containing a nitroglycerindeterrent mixture. The powder grains are attacked by the casting solvent and the individual grains converted therewith without the application of pressure into a homogeneous propellent grain conforming to the dimensions and shape of the casting container.

Powder grains which have been case hardened offer distinct advantages over untreated extruded powder grains particularly when forming propellent grains of great length. If the grains have not been treated to case harden the surfaces thereof, the casting solution rapidly attacks the first grains contacted and converts them into a coherent mass which retards and in most instances blocks the flow of solvent and thus prevents the addition of sufficient solvent to contact those grains relatively far removed from the place of entrance of the solution. It therefore becomes impractical or impossible to consolidate the complete charge into a coherent homogeneous mass. Solvent attack on propellant grains having a surface which has been treated with a solvent in accordance with the process of this invention, on the other hand, is retarded and the foregoing disadvantage inherent with a process utilizing untreated grains is thus eliminated. None of the prior art extrusion processes of forming propellent grains are capable of-producing grains of great length while a substantially homogeneous grain several feet in length can be prepared in accordance with this invention.

As a specific example of one embodiment of this invention, a nitrocellulose powder composition containing about parts by weight nitrocellulose, 9 parts dinitrotoluene and one part ethyl centralite Which has been sub jected to a series of kneading operations while moistened with ether-alcohol solvent mixture in accordance with usual practice to effect uniformity of the composition is expressed through a suitable die to form cylindrical grains thereof having a diameter of about 0.03 inch and is cut into lengths of about 0.03 inch. These grains are then dried for-about 24 hours at about 50 C. or until the moisture content thereof has been reduced to about 0.5 percent.

About 67 parts by weight of the extruded grains thus obtained are suspended in about 252 parts water containing about 5 parts gum arabic. About 34 parts ethyl acetate are added to the water suspension and the slurry agitated at a temperature of about 70 C. for about minutes at which point the surface of the powder grains has been attacked and softened by the ethyl acetate. The temperature of the slurry is then gradually elevated to about 99 C. and maintained at this temperature until substantially all of the ethyl acetate has been removed therefrom. After substantially all of the solvent has been removed, the powder grains are separated from the water by filtering and dried to a total volatiles content of about one percent by heating at about 60 C. for about 12 hours.

The contour and dimensions of the extruded powder grains treated in accordance with the foregoing steps in the process of this invention are riot altered sufi'iciently to interfere with subsequent casting. The dried grains are transferred to a cylindrical container having one closed end which is about one foot in diameter, about 10 feet in length and a laminated wall thickness of about 0.15 inch formed from cellulose acetate sheets bonded together by treating with acetone. The charged container is shaken to compact the powder grains therein to their maximum packing density and about 37 parts castrent or plasticizer and about one part stabilizer.

ing solvent containing approximately 74 percent nitroglycerin, about 25 percent triacetin and about one percent ethyl centralite is delivered to the mold and forced into the voids between the powder grains displacing the air therefrom. The container charged with powder grains dispersed in the casting solvent is then stored at approximately 70 C. for about 100 hours. At the end of this period of time, the powder grains and casting solvent have formed a coherent homogeneous grain of propellent powder which is confined within a desirable plastic covermg.

While embodiments of this invention have been described in detail in the foregoing, it is to be understood that this invention is not limited thereto. For example, protective colloids other than gum arabic such as animal glue, corn starch or the like can be utilized to prevent the powder grains from coagmenting during the case hardening treatment. Likewise, other solvents for nitrocellulose which are immiscible with water and can be satisfactorily removed from the slurry, for instance, methyl ethyl ketone alone or in combination with toluene, isopropyl acetate ethyl formate, methyl isobutyl ketone or the like can be utilized as the solvent in the case hardening process. In addition, the composition of the casting solvent can be from about 64 parts to about 84 parts nitroglycerin, about 15 to about 35 parts deter- When deterrents which are relatively good solvents for nitrocellulose are utilized, lesser amounts of nitroglycerin may be included when so desired to obtain specific ballistic properties. Ordinarily the ratio of casting solvent to extruded grains will be from 0.30 part to 0.60 part solvent to one part powder grains. Any temperature can be utilized to convert the extruded grains into a homogeneous grain with nitroglycerin provided that temperatures sufficiently high to effect firing of the explosive should be avoided and the time at such temperatures should be sufiiciently long to permit permeation of the casting solvent throughout the extruded grains and result in a substantially homogeneous solid solution. Other suitable deterrents or plasticizers may be utilized, for example, dibutylphthalate, dimethylphthalate, dinitr-otoluene, diglycol diacetate, dimethyl sebacate, or the like, while any suitable stabilizer such as diphenylamine or methyl centralite, mixtures thereof, or the like can be substituted for the ethyl centralite in the foregoing embodiments. While this invention has been described as particularly advantageous with respect to the casting of giant pro pellent grains of great length, it obviously can be utilized to advantage in forming smaller powder grains. This invention also contemplates the incorporation of modifiers such as carbon black, lead compounds and the like in the grains if required to impart certain ballistic properties thereto. In the foregoing embodiment, the propellant grain Was cast in a plastic container which is particularly advantageous in those instances wherein a coating of this type is desired on the grain. However, any type of mold such as one made from metal can be utilized and the grains can be formed around plastic inserts to lend greater strength thereto or to give specific grain geometries if desired. Casting solvents other than nitroglycerin such as, for example, nitroglycols, deterrents, and the like may be utilized depending upon the ballistic characteristics desired.

It is apparent from the foregoing description that this invention accomplishes its objects and provides an improved method for making giant propellent grains and also provides an improved process for forming substantially homogeneous double base propellent grains from nitroglycerin and single base propellent grains of lesser dimensions.

Having described the invention, what is claimed and desired to be secured by Letters Patent is:

1. In the manufacture of propellants, the process which comprises forming nitrocellulose base propellent grains by an extrusion process, softening the surface of the grains by means of a solvent for nitrocellulose and thereafter removing the solvent therefrom, mixing the resulting case hardened grains with a solvent for nitrocellulose and consolidating this mass into a propellent grain by the dissolution of the smaller grains by the solvent.

2. A process for making propellent powder grains from smaller colloided nitrocellulose base powder grains formed by conventional extrusion methods which comprises in combination the steps of softening the surface of the smaller grains with a solvent for nitrocellulose while the grains are suspended in a two phase system containing said solvent, a protective colloid and a non-solvent for nitrocellulose, removing the solvent from the grains by evaporation and separating the resulting case hardened grains from the non-solvent, and placing said grains in a mold suitable for shaping powder grains, flowing a solvent for nitrocellulose into the mold until the interstices therebetween are filled therewith, and thereafter maintaining the mass in the mold at atmospheric pressure until solidification thereof into a propellent powder grain.

3. A process for making propellent powder grains from smaller colloided nitrocellulose base powder grains formed by conventional extrusion methods which comprises in combination the steps of case hardening the surface of said smaller grains by softening the surface thereof with a solvent for nitrocellulose while the grains are suspended in a two phase system containing said solvent, a protective colloid and a non-solvent for nitrocellulose, removing the solvent from the grains byevaporation and separating the resulting case hardened grains from the non-solvent, and placing said grains in a mold suitable for shaping powder grains flowing a solvent having the composition of from about 64 to about 84 parts nirtoglycerine, from about 15 to about 35 parts deterrent and about 1 part stabilizer into the mold until the interstices therebetween are filled therewith, and thereafter maintaining the mass in the mold at atmospheric pressure until solidification thereof into a propellent powder grain.

4. A process for making propellent powder grains from smaller colloided nitrocellulose base powder grains which were formed by conventional extrusion methods which comprises in combination the steps of case hardening the surface of said smaller grains by softening the surface thereof with a solvent for nitrocellulose while the grains are suspended in a two phase system containing said solvent, a protective colloid and a non-solvent for nitrocellulose, removing the solvent from the grains by evaporation and separating the resulting case hardened grains from the non-solvent, and placing from about 0.3 part to about 0.6 part of the resulting case hardened smaller grains in a mold suitable for shaping larger powder grains, compacting the smaller powder grains therein to their maximum packing density, flowing about one part casting solvent containing in its composition about 74% nitroglycerine, about 25% triacetin and about 1% ethyl centralite into the mold until the interstices are filled therewith, and thereafter maintaining the mass in the mold until solidification thereof into a propellent powder grain.

5. A process for making propellent powder grains which comprises in combination, the steps of forming colloided nitrocellulose base powder grains by an extrusion process, suspending the powder grains in a non-solvent medium containing a protective colloid, adding to the suspension a solvent for nitrocellulose which is substantially immiscible with the non-solvent medium, agitating the suspension until the surface of the grains is softened by the solvent, removing said solvent from the grains and separating the resulting case hardened grains from the suspending medium, placing said grains in a mold suitable for shaping powder grains, flowing a solvent for nitrocellulose into the mold until the interstices between the grains are filled therewith, and thereafter maintaining the mass in the mold until solidification thereof into a propellent powder grain.

6. In a process for making propellent powder grains from smaller colloided nitrocellulose base powder grains which involves placing said smaller grains in a mold and thereafter flowing a solvent for the grains into the interstices therebetween, the method of retarding the rate of the dissolving action of said solvent on said grains in order to prevent the flow of said solvent into the mold from being blocked by the coherent mass formed by solution of the grains in the solvent, which process comprises caes hardening the surface of the smaller grains prior to placing them in the mold by softening the surface thereof with a solvent while the grains are suspended in a two phase system containing a solvent for nitrocellulose, a protective colloid and a non-solvent, removing the solvent References Cited by the Examiner UNITED STATES PATENTS 2,027,114 1/ 36 Olsen et al. 2,120,324 6/38 Dickerman. 2,333,637 11/43 Carey.

3/47 Silk et al.

CARL D. QUARFORTH, Primary Examiner.

WILLIAM G. WILES, Examiner. 

1. IN THE MANUFACTURE OF PROPELLANTS, THE PROCESS WHICH COMPRISES FORMING NITROCELLULOSE BASE PROPELLENT GRAINS BY AN EXTRUSION PROCESS, SOFTENING THE SURFACE OF THE GRAINS BY MEANS OF A SOLVENT FOR NITROCELLULOSE AND THEREAFTER REMOVING THE SOLVENT THEREFROM, MIXING THE RESULTING CASE HARDENED GRAINS WITH A SOLVENT FOR NITROCELLULOSE AND CONSOLIDATING THIS MASS INTO A PROPELLENT GRAIN BY THE DISSOLUTION OF THE SMALLER GRAINS BY THE SOLVENT. 